United States Patent Application Publication 2004/0209150, dated Oct. 21, 2004, and titled “Stamped Fuel Cell Bipolar Plate” describes an assembly of structural and functional elements used in a gaseous fuel (e.g., hydrogen)/gaseous oxidant (air) fuel cell. The elements for many electrochemical cells are stacked to form a multi-cell assembly for producing electrical power at specified electrical current and potential (voltage).
The interior cells of the stacked assembly comprise one side of each of two opposing bipolar plates. The facing bipolar plates enclose cell elements comprising a proton exchange membrane-electrode assembly, gaskets, gas diffusion media, and the like. Each bipolar plate is formed of two like-shaped plates, in face-to-face arrangement, that have gas flow passages on their external faces and internal coolant passages defined by their inverse and facing sides. One side of a first bipolar plate provides passages for the flow of hydrogen to the anode side of the membrane-electrode assembly and one side of a second, opposing bipolar plate provides passages for the flow of air to the cathode side of the membrane-electrode assembly. Heat is produced in the operation of the stack of cells and coolant flow through the interior of the bipolar plates is used to cool the stack, particularly the internal cells of the stack.
The plates are typically stamped from a thin, generally rectangular sheet of metal and, preferably, each sheet is of generally the same shape. Opposing edges at the short sides of each rectangular sheet are shaped, respectively, for the inlet and exit of fuel, oxidant, and coolant. The central portion of each sheet is shaped with spreading channels for gas flow from the inlet, channels for distributed flow of gas over the membrane, and converging channels for directing gas to the exit. When two such sheets are suitably bonded with gas flow passages facing outwardly to form a bipolar plate, the inverse sides of the stamped sheets provide flow-controlling passages for the coolant.
In the assembly of a fuel cell stack each bipolar plate is intended to form a part of two adjacent cells, one cell on each outer face of the bipolar plate. One outer face of a bipolar plate provides an anode plate for one cell and the other outer face provides a cathode plate for the adjacent cell. However, the bipolar plates at the ends of the stack have no cells on their outer plate surfaces. Accordingly, these plates are sometimes called unipolar plates. Although unipolar plates have the same structure as bipolar plates, they may not require the coolant flow rate permitted by the bipolar plate structure. And the bipolar plates toward the end of the stack may not require high coolant flow rates either.
When possible, it is preferred to use a single set of stamping tools for forming each of the metal sheets used in pairs in making bipolar plates. However, this means that the coolant flow in each bipolar plate may be the same regardless of its ultimate position in a stack of plates. It is desirable to have a way of altering coolant flow in selected bipolar (or unipolar) plates without requiring additional stamping tools.